Handover (HO) is a procedure in which a mobile station (MS) moves from an air interface of one base station to an air interface of another base station. A handover procedure of a general IEEE 802.16e system is described below.
In an IEEE 802.16 network, a serving base station (SBS) may broadcast neighbor base station information through a neighbor advertisement (MOB_NBR-ADV) message in order to inform a mobile station (MS) of basic network configuration information (topology).
The MOB_NBR-ADV message includes system information of the serving base station and neighbor base stations, for example, preamble indexes, frequencies, handover (HO) optimization possibility, downlink channel descriptor (DCD)/uplink channel descriptor (UCD) information, etc.
The DCD/UCD information includes information required for an MS to perform information exchange in downlink and uplink. For example, the DCD/UCD information includes HO trigger information, a medium access control (MAC) version of a base station, Media Independent Handover (MIH) capability information, etc.
A general MOB_NBR-ADV message includes only information associated with neighbor base stations of an IEEE 802.16e type. Information associated with base stations of the type other than an IEEE 802.16e type may be broadcast to MSs through a service identity information advertisement (SII-ADV) message. Accordingly, an MS may acquire information associated with base stations of a heterogeneous network by requesting an SBS to transmit the SII-ADV message.
A procedure for performing handover by an MS, which has acquired information associated with neighbor base stations using the above-described method, in an IEEE 802.16e network is described below in more detail with reference to FIG. 1.
FIG. 1 illustrates an exemplary handover procedure which may be performed in an IEEE 802.16e system.
As shown in FIG. 1, first, a mobile station (MS) may access a serving base station (SBS) to perform data exchange (S101).
The SBS may periodically broadcast information associated with neighbor base stations (BSs) located within the coverage of the SBS to the MS through an MOB_NBR-ADV message (S102).
The MS may begin to scan candidate HO BSs using an HO trigger condition while communicating with the SBS. The MS may transmit a handover request (MOB_MSHO-REQ) message to request the SBS to perform a handover procedure when a handover condition is satisfied, for example, when a predetermined hysteresis margin value is exceeded (S103).
The SBS may inform candidate HO BSs included in the MOB_MSHO-REQ message of the handover request received from the MS through an HO-REQ message (S104).
The candidate HO BSs may take precautions for the MS which has requested handover and transmit information associated with handover to the SBS through an HO-RSP message (S105).
The SBS may transmit the information associated with handover acquired from the candidate HO BSs through the HO-RSP message to the MS through a handover response (MOB_BSHO-RSP) message. The MOB_BSHO-RSP message may include information required to perform handover such as an action time for handover, a handover identifier (HO ID) and a dedicated HO CDMA ranging code (S106).
The MS may determine one target BS (TBS) among the candidate HO BSs based on information included in the MOB_BSHO-RSP message received from the SBS. Then, the MS may attempt to perform ranging by transmitting a CDMA code to the determined target BS (S107).
The target BS which has received the CDMA code may transmit information indicating whether or not ranging has succeeded and physical correction values to the MS through a ranging response (RNG-RSP) message (S108).
Then, the MS may transmit a ranging request (RNG-REQ) message for authentication to the target BS (S109).
The target BS which has received the ranging request message of the MS may provide system information such as a connection identifier (CID) which can be used in the BS to the MS through a ranging response message (S110).
When the target BS has successfully completed authentication of the MS and has transmitted all update information, the target BS may notify the SBS of the MS of whether or not handover has succeeded through a handover completion message (HO-CMPT) (S111).
Thereafter, the MS may exchange information with the target BS which performs handover (S112).
A handover procedure that may be performed in the IEEE 802.16m system is similar to the handover procedure of the IEEE 802.16e system described above. However, messages having different names may be defined as follows.
MOB_NBR-ADV->AAI_NBR-ADV: This message includes system information or the like that is transmitted in the form of an S-SFH rather than a DCD/UCD.
MSHO-REQ->AAI-HO-REQ
BSHO-RSP->AAI-HO-CMD
RNG-REQ (CDMA code)->Ranging preamble code
RNG-RSP (ranging status)->AAI-RNG-ACK (ranging status)
RNG-REQ (MAC message)->AAI-RNG-REQ
RNG-RSP->AAI-RNG-RSP: This message includes a TSTID or an STID which is a station ID rather than a CID.
An Advanced Mobile Station (AMS) that complies with the general handover procedure described above performs handover according to channel status or coverage of an Advanced Base Station (ABS) as the AMS moves through an overlay network. However, when the AMS moves in a region in which ABSs (for example, a femto cell, a pico cell, a relay station, or the like) whose coverage is relatively small are present in the coverage of a macro ABS, handover triggering often occurs.
Although the AMS may hand over from the macro ABS to a small ABS having high signal quality, the AMS may again perform handover due to small coverage of the target ABS. Such frequent handover is undesirable not only in terms of latency of the AMS but also in terms of resource management of the ABS since handover of the AMS requires coordination of the ABS.
Therefore, there is a need to provide a method for preventing unnecessary handover in a scanning procedure or a handover triggering procedure in a situation in which ABSs having different coverage are present together as described above.